A communications system typically involves transmitting a sequence of data from a communications transmitter to a communications receiver over a communications channel. The communications transmitter and/or the communications receiver may include one or more cable modems, set-top boxes, mobile radios, laptop modems, and/or cellular telephones to provide some examples. The communications channel may include a microwave radio link, a satellite channel, a fiber optic cable, or a copper cable to provide some examples. The communications channel contains a propagation medium that a transmitted communications signal passes through before reception by the communications receiver. The propagation medium of the communications channel introduces interference and/or distortion into the transmitted communications signal causing a received communications signal to differ from the transmitted communications signal. The communications channel may introduce additional interference and/or distortion resulting from undesirable signals and/or noise into the transmitted communications signal. The communications transmitter and/or the communications receiver may introduce further interference and/or distortion into the transmitted communications signal causing the received communications signal to differ from the transmitted communications signal.
Communications systems may use an adjustable filter in the form of an interference cancellation filter to reduce the effect of the interference and/or distortion attributable to the communications channel, the communications transmitter, and/or the communications receiver. To compensate for this interference and/or the distortion, a conventional interference cancellation filter may adaptively adjust an impulse response by updating interference cancellation filter coefficients through, for example, a least-squares algorithm, such as the widely known Least Mean Squared (LMS), Recursive Least Squares (RLS), Minimum Mean Squared Error (MMSE) algorithms or any suitable equivalent algorithm that yields a least-squares result. Alternatively, the conventional interference cancellation filter may employ techniques disclosed in
(i) U.S. patent application Ser. No. 10/142,189, filed May 8, 2002, entitled “Cancellation of Interference in a Communication System with Application to S-CDMA,” now U.S. Pat. No. 7,110,434;
(ii) U.S. patent application Ser. No. 10/242,032, filed Sep. 12, 2002, entitled “Successive Interference Canceling for CDMA,” now U.S. Pat. No. 7,190,710;
(iii) U.S. patent application Ser. No. 10/136,059, filed Apr. 30, 2002, entitled “Chip Blanking and Processing in SCDMA to Mitigate Impulse and Burst Noise and/or Distortion,” now U.S. Pat. No. 7,236,545;
(iv) U.S. patent application Ser. No. 10/000,415, filed Nov. 2, 2001, entitled “Detection and Mitigation of Temporary Impairments in a Communications Channel,” now U.S. Pat. No. 7,308,050;
(v) U.S. patent application Ser. No. 10/962,803, filed Oct. 12, 2004, entitled “Chip Blanking and Processing in SCDMA to mitigate impulse and burst noise and/or distortion,” now U.S. Pat. No. 7,366,258;
(vi) U.S. patent application Ser. No. 11/089,139, filed Mar. 24, 2005, entitled “Cancellation of Burst Noise in a Communication System with Application to S-CDMA,” now U.S. Pat. No. 7,415,061; and
(vii) U.S. patent application Ser. No. 10/237,853, filed Sep. 9, 2002, entitled “Detection and Mitigation of Temporary (Bursts) Impairments in Channels using SCDMA,” now U.S. Pat. No. 7,570,576, each of which is incorporated herein by reference in its entirety.
These conventional interference cancellation approaches use, in part, a scheme tantamount of time-averaging to determine correlation properties of the interference and/or distortion. Commonly, the interference and/or distortion introduced by the communications channel may include one or more time-varying conditions. The conventional interference cancellation filter may not properly determine the correlation properties of the interference and/or distortion in the presence of the one or more time-varying conditions. In this situation, the least-squares algorithm or the suitable equivalent may inadequately train the conventional interference cancellation filter to compensate for the interference and/or distortion. To provide some robustness against time-varying interference, conventional cancellation or mitigation techniques have added some nonlinear processing to the time-averaging results. For example, conventional nonlinear processing may include taking frequency bins with relatively large interference power for a brief time, and holding a higher power value. Still, the result of these conventional techniques is a single time-average capture of the interference characteristics, but modified from the true time-average in a manner to provide robustness against dimensions of interference which occasionally incur relatively large power. The result of nonlinearly treating the time-averaging, and introducing larger-than-averaged interference estimates in some dimensions, is that when extremely powerful interference is present it will be greatly reduced as it appears at a decision point in the receiver. However, typically this processing also incurs the property that when the interference is not extreme, the noise power at a decision point will be increased compared to the noise which would be at the decision point with true time-averaging. Thus, while these prior techniques have been very successful, still, the solution provides robustness in the presence of the occasionally extreme interference in one or more dimensions, but at the sacrifice of some performance when one or more of the dimensions is over-protected
Therefore, what is needed is an interference cancellation filter that is capable of compensating for the interference and/or distortion in the presence of the one or more time-varying conditions, without incurring a penalty when the interference is benign.
The present invention will now be described with reference to the accompanying drawings. In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements. The drawing in which an element first appears is indicated by the leftmost digit(s) in the reference number.